ext4: refuse to create ea block when umounted

The ea block expansion need to access s_root while it is
already set as NULL when umount is triggered. Refuse this
request to avoid panic.

Reported-by:  <syzbot+2dacb8f015bf1420155f@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?id=3613786cb88c93aa1c6a279b1df6a7b201347d08
Link: https://lore.kernel.org/r/20230103014517.495275-3-jun.nie@linaro.org


Cc: stable@kernel.org
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>